1. Field of the Invention
The present invention relates to a method of production of a multi-layered steel pipe as well as to a multi-layered steel pipe produced from helical welding seam pipes.
2. Brief Description of the Background of the Invention Including Prior Art
It is known that pipes with a laminatad, sheet, or layer construction can withstand high internal pressures. Therefor, frequently two inserted pipes are connected to a multi-layer pipe by drawing such as taught for example in German Patent Application Laid Open No. DE-OS 2,501,156 and in U.S. Pat. No. 4,125,924. Based on the technical manufacturing requirements this method is limited to small pipe diameters with homogeneous surfaces.
Another possibility of producing multi-layer pipes is taught in German Patent Application Laid Open No. De-OS 3,000,665, where the inner tube is expanded by way of a steel ball being pressed through the pipe. Again, this method allows only to produce multi-layer pipes having a small diameter with a smooth inner surface.
In order to produce high pressure pipelines with large diameters production has been changed over some time ago to pipes with strengthening bands as taught in German Patent No. DE-PS 936,981, where the pipe was expanded into loose circumferentially disposed girths by way of pressurized liquid. This process carries the disadvantage that the pipe can bulge out between the strengthening bands based in many cases on unavoidable small wall thickness deviations or other flaws or unsound material sections. Thus no uniformly constant pipe diameter is produced.
This disadvantage is avoided according to a method according to the German Patent No. DE-PS 548,576. This patent teaches a production process for longitudinally welded high pressers pipes of large diameter in multiple layer construction, which are provided with long running longitudinal welded seam jackets instead of strengthening bands, and the pipes are expanded by liquid pressure. This system at the same time employs the effect of material improvement by cold-working, but it is associated with the decisive disadvantage that the tube pieces protruding beyond the jacket have to be cut off, which renders this multi-layer pipe uneconomical.
A similar process is known from the article "Hooped" pipe, Pipeline and Gas Journal, November 1973. One might see with hindsight that a corresponding not described provision of a hydraulic expansion process avoids the above disadvantage of cutting off the ends. This process allows to produce work-hardened high pressure pipes for example for gas pipes up to a diameter of 1422 millimeters (56 inches) and wall thicknesses of 16 millimeters (0.63 inches) with a yield strength of the material of up to about 412 Newton per square millimeter for the pipe to be expanded.
Larger pipe diameters and larger operating pressures than about 45 bar were not believed to be possible, that is requirements, which are today standard for gas pipelines with operating pressures of 70 bar and which are expected to rise to 100 bar. This system in addition carries the disadvantage that the outer jacket pipes do not correspond to the length of the inner pipes and that therefor the finished multi-layer pipe is not accessible to a usual water pressure and non-destructive testing of the complete pipe body. In addition, the system requires a special strengthening with bands of the pipeline in the area of the butt welding connections, such as is taught for example in the French Pat. No. FR-PS 2,262,246. This method is opposed to a far reaching automation of the butt welding process with the usual equipment employed by pipe laying organizations. Furthermore, there are expected to exist problems with the adhesion of the outer pipe insulation, which is always required with pipeline pipes based on the danger of corrosion of steel pipes. It was indicated above that there are disadvantages connected with hydraulic expansion in case of flaws and unsound regions in the work piece to be processed.
Another disadvantage of this system is associated with the employment of longitudinal seam pipes. Based on their ovality and their nonevenness the inner pipes have to be rounded before being inserted into the outer pipes as can be gathered from a further publication relating to this system entitled "Hooped Pipe"-Pipe Line Industry, January 1975. According to this further development in addition pipes are to be produced with a diameter of up to 2 meter as longitudinal seam pipes in pressure stages of up to 150 bar, however based on the necessarily employed lengths of pipe in connection with three roller bending machines uneconomically short lengths result. The expanding is here possible with a liquid pressure of up to 90 percent of the yield stress limit of the outer pipe, where however the initial yield stress of the inner pipe cannot exceed 412 Newton per square millimeter.
However, variants of this systems provide two advantages which are employed in the present invention:
Materials of different wall thickness and strength can be employed for the outer and, respectively, inner pipe.
After the expansion pressure stresses are generated in the inner pipe, the advantages of which will be disclosed in the present invention, but these pressure stresses were not taught in the reference.
There is known a further method for the production of multi-layer steel pipes according to European Patent Disclosure No. EP-AL-0015712, where the outer pipe is heated and the inner pipe is undercooled such that a diameter difference results, which allows an insertion of the pipes. Then the temperature equalization is obtained under simultaneous hydraulic expansion of the inner pipe. Apart from the fact that this process appears to be only usable for two layered pipes, the high energy requirements appear to be justified only for the otherwise problem plagued connection of a highly alloyed or stainless steel pipe as an inner pipe with a less expensive steel as an outer pipe.
A big problem has proven to be the safety against crack formation and crack propagation at the expansion of the inner pressures in gas pipelines to more than 70 bar. It is known that cracks propagate in steel pipe with a speed of up to 330 meters per second and that they destroy several kilometer of pipe completely. The state of the art recited up to now has not provided any solution for this problem.
A large number of proposals have been made as to the local limiting of such cracks such as for example by way of winding steel cables around regular pipe lines as taught in German Petit Patent No. DE-GM 7,709,311. These windings interfere substantially with the laying of the pipe and its installation at gas pipes with typical dimensions of 1420 millimeter diameter and wall thicknesses of 17.5 millimeter at a distance of only from 9 to 200 meter and widths of up to two meter.
For this purpose either the pipe insulation is damaged if the cable is wound on top of it or the insulation of the winding has to be performed manually upon interruption of the automatic insulating process.
It is known that helical welding seam pipes are provided with a substantially larger resistance against crack propagation and crack formation as compared with longitudinally welded pipes based on the high notched bar impact strength in the main stress direction, which is the circumferential direction of the pipe, which depending on the seam helix angle is either equal or slightly below the value for the direction of rolling of a steel strip. This effect is employed according to U.S. Pat. No. 3,698,746 by inserting a helical welding seam pipe into a regular one wall pipeline as a crack propagation stopper. However, the advantages of a laminate type construction at a multi-layer steel pipe are not disclosed.
Based on other advantages, which also become effective according to the present invention, of the helical welding seam steel pipes, where economic steel strip of large lengths is employed in a continuous production process, the production of multi-layer high pressure tubes has already been proposed by simultaneous forming and welding of several steel strips disposed on top of each other, compare German Patent Laid Out Nos. DE-AS 1,944,587, DE-AS 1,963,805, DE-AS 2,745,389 and corresponding U.S. Pat. No. 4,247,033. Apart from the large equipment requirements this kind of production allows only to a very limited extent the generation of a pressure stress in the inner pipe, and practical experience has even shown that in the case of the teaching of No. DE-AS 1,963,805 necessarily and unavoidably gaps are produced between the pipe layers.
This internal pressure stress is of course desired in the laminate construction.
Mechanical expanders are known which are capable of expanding a pipe depending on the length, that is by a certain fixed amount. In the case of the production of helical welding seam pipes with welding seam reinforcements there are known specially adapted aggregates according to German Patent Applications Laid Open Nos. DE-OS 2,627,172 and DE-OS 2,641,051, which also are useful according to the present invention.